This invention generally relates to cylindrical batteries that utilize a tubularly shaped separator to contain and physically separate the battery""s electrodes from one another. More particularly, this invention is concerned with a separator having a free standing portion that maintains its structural rigidity when wetted by the battery""s electrolyte.
Cylindrically shaped batteries are suitable for use by consumers in a wide variety of devices such as flashlights, radios and cameras. Batteries used in these devices typically employ a cylindrical metal container to house two electrodes, a separator, a quantity of electrolyte and a closure assembly. Typical electrode materials include manganese dioxide as the cathode and zinc as the anode. The zinc is commonly employed in particulate form suspended in a gel. An aqueous solution of potassium hydroxide is a common electrolyte. A separator, conventionally formed from one or more strips of paper, is positioned between the electrodes. The electrolyte is readily absorbed by the separator and gelling medium.
One of the issues that battery manufacturers must address is the requirement that direct contact between the anode and cathode within the battery be prevented. If the anode and cathode are allowed to physically contact one another, a chemical reaction takes place and the useful electrochemical capacity of the battery is reduced. The function of the separator is to prevent direct contact between the anode and cathode while allowing for ionic conductivity therebetween.
Small cylindrical batteries must be manufactured to withstand the physical rigors associated with the manufacturing and distribution processes as well as the handling of batteries by consumers. In particular, batteries must be able to withstand being accidentally dropped by consumers. In batteries with a semi-fluid electrode, such as the gelled anode used in many cylindrical alkaline batteries, the drop may cause a small portion of the anode to fragment and thereby break free from the rest of the anode. The fragmented anode must be prevented from coming into direct contact with the cathode. Similarly, if the cathode is hard and susceptible to fragmentation when the cell is dropped, small fragments of the cathode may become separated from the body of the cathode and need to be contained. As shown in FIG. 1, many conventional cell constructions have addressed this problem by using an elastomeric seal 78 that includes a V-shaped leg 80 that projects toward the interior of the cell and contacts the top of the coiled separator 20 thereby forming a barrier that prevents anode fragments 64 from contacting the cathode 54. However, the V-shaped projections on the conventional seal designs occupy a portion of the cell""s internal volume which could be better used to hold an additional quantity of the cell""s electrochemically active materials. Consequently, many cell designs have been proposed that utilize low volume seal designs which do not cooperate with the separator to form a barrier that isolates the anode from the cathode. Unfortunately, eliminating that portion of the seal that helps to contain the anode has led to an increased level of internal electrical shorting between the anode and cathode when cells are dropped by consumers. The internal shorting problem is due to the free standing portion of the separator, located above the anode/cathode interface, losing its stiffness when it absorbs some of the cell""s electrolyte and then collapsing away from the low volume seal so that an unobstructed path is created between the anode and the cathode. As shown in FIG. 2, the collapsed portion 46 of the separator 20 has allowed a fragment 64 of anode 66 to contact cathode 54.
Therefore, there exists a need for a separator that will be structurally self supporting after absorbing the cell""s electrolyte such that the separator can contain even small fragments of an electrode thereby preventing the formation of an internal electrical short circuit.
The present invention provides an electrochemical cell with a separator having a free standing reinforced portion that is structurally self supporting even after absorbing some of the cell""s electrolyte. The separator is reinforced by coating an edge of the separator so that the reinforced edge prevents fragmented pieces of one electrode from contacting the cell""s opposing electrode. The coating material used to reinforce a portion of the separator occupies very little volume within the cell.
In one embodiment, the electrochemical cell of the present invention includes a container having an open end, a closed end and a sidewall. A first electrode is located within the container and defines a cavity having an interior surface. Electrolyte is located within the container and contacts the first electrode. A separator forms a lining on the interior surface of the cavity defined by the first electrode. The separator has a free standing reinforced edge that extends beyond the first electrode toward the open end of the container. The reinforced edge has a reinforcing material that provides structural support to the edge of the separator after the separator has absorbed a portion of the electrolyte. A second electrode is located within the separator cavity. The separator forms an interface between the first and second electrodes. A closure assembly is secured to the open end of the container.
The present invention also relates to a process for manufacturing a cell. The process includes the following steps. Providing a strip of separator material. Coating at least one edge of the separator with a reinforcing material. Coiling the coated separator strip to form a tube having a noncoated portion and a coated reinforced edge defining an opening at one end of the tube. Providing a container with an open end and including a first electrode defining a cavity therein. Inserting the coiled tube into the cavity defined by the first electrode so that the noncoated portion contacts the first electrode and a coated edge of the tube extends beyond the first electrode toward the open end of the container. Inserting a second electrode into the tube defined by the coiled separator. Closing the container by securing a closure assembly to the open end of the container.
The present invention also relates to another process for manufacturing a cell. This process includes the following steps. Providing a separator material. Coiling the separator to form a tube. Coating an edge of the tube with a reinforcing material. Providing a container having an open end, a closed end and a first electrode defining a cavity. Inserting the coiled coated separator tube into the cavity so that the open end of the tube is in close proximity to the open end of the container and the opposite end of the tube is in close proximity to the closed end of the container. Inserting a second electrode into the tube defined by the coiled separator. Closing the container by securing a closure assembly to the open end of the container.
The present invention relates to yet another process for manufacturing a cell. This process includes the following steps. Providing a container that has an open end, a closed end and a first electrode located within the container. The first electrode defines a cavity. Providing a first rectangularly shaped strip of separator that has two opposing edges coated with a reinforcing material. Inserting the coated strip of separator into the cavity defined by the first electrode so that uncoated portion of the separator lines the cavity and the coated edges extend beyond the first electrode toward the open end of the container. Inserting a second electrode into the separator lined cavity. Closing the container by securing a closure assembly to the open end of the container.